The present invention relates to a surface inspecting apparatus for transversely scanning the surface of a web material continuously moving lengthwise to detect surface defects of the web material so as to qualify or evaluate the surface condition of the web material.
Various apparatus are well known for scanning a surface of a continuously moving web material, such as films, paper sheet, thin plates or the like, with a scanning beam in the direction of the width of the web material to detect surface defects of the web material for evaluating or qualifying the web material. Some of these surface scanning apparatus utilize a photo-electric detector which detects light modulated by surface defects of the web material to provide surface defect signals. Based on the surface defect signals a judgment is made as to whether the web material has unacceptable surface defects. Because they are non-contact devices and permit high speed surface defect detection, these surface inspection apparatus can be widely used in a web material production line.
One such surface inspection apparatus utilizes a filtering circuit for filtering out noises included in detected surface signals, the filtered surface signals being sent to a binary signal generating circuit. The binary signal generating circuit compares the surface signal with a predetermined threshold level (THLD) to transform it into a binary signal.
Surface signals modulated by a web material having a surface pattern non-uniform or irregular in the transverse direction of the web material often contain unusual noises regularly generated during repeated transverse scans. Therefore, the threshold level should be higher than a maximum level of such regularly generated unusual noises which is relatively high. Therefore, only surface defects which provide defect signals having signal levels higher than the threshold level can be detected.